Continuous flow synthesis of nanomaterials using ionic liquids in microfluidic reactors

ABSTRACT

A method for manufacturing metal nanoparticles includes the use of a microfluidic device. The microfluidic device has a first channel having a first inlet; a second channel having a second inlet; a third channel having a third inlet; and a main channel having a main inlet and an outlet. The first channel, second channel, and third channel all lead into the main channel. The method involves injecting a solution of a metal/ligand into the first inlet, injecting a solution of a reducing agent into the second inlet, injecting a solvent comprised of an ionic liquid into the third inlet, and injecting an inert carrier into the main inlet. The solution of the metal/ligand, the solution of the reducing agent, the solvent and the inert carrier are combined together in the main channel, and the metal/ligand and the reducing agent are reacted for a time sufficient to form a metal nanoparticle.

CROSS REFERENCE TO PROVISIONAL APPLICATION

The present application is based upon and claims the benefit of priority from Provisional U.S. Patent Application No. 61/544,447 (Attorney docket No. 28080-679) filed on Oct. 7, 2011, the entire contents of which are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Grant No. CMMI-0926969, awarded by the National Science Foundation. The Government has certain rights in the invention.

BACKGROUND

There is a rapidly growing demand for metal nanoparticles. Nanoparticles are used in a wide variety of industries, such as pharmaceuticals, renewable energy, textiles, and cosmetics. The global market was projected to be $220 billion in 2010, with 61% attributable to fabrication. There is a critical need for innovative nanomanufacturing approaches that minimize energy use, emissions, waste, all while maximizing the quality of the product.

However, nanoparticle manufacturing is still commonly performed on a batch-by-batch, lab-scale process. Control over these variables is difficult in large-scale batch reactors because of limitations in heat and mass transport. As such, the scalability of this method is limited and mitigation of these limitations translates to higher costs. Nanoparticles may also be produced in small volume batch reactions although difficulties controlling and reproducing size, size distribution, and morphology are typical. As novel applications for nanoparticles continue to emerge, there is an increasing need for approaches to nanomanufacture these materials using inexpensive, rapid, and reproducible methods that have minimal impact on the environment.

Reactor miniaturization via microfluidic technology has enabled the continuous flow synthesis of a large number of molecules and nanomaterials. Microfluidic reactors offer several advantages over traditional batch scale syntheses; namely, continuous throughput, superior reaction control, and minimal solvent waste and byproduct generation. Such control is made possible by improved heat and mass transport within the microfluidic channels that result from high surface area-to-volume ratios, in addition to fast reagent mixing. Moreover, microfluidic devices allow for reduction of environmental risks associated with nanofabrication by allowing small-volume, on-demand syntheses that also result in higher yields and less by-product generation.

SUMMARY

The present disclosure is directed toward a method to fabricate monodisperse metal nanoparticles using a simple continuous flow microfluidic device in tandem with imidazolium-based ionic liquids.

In one example of the present disclosure, a method for manufacturing metal nanoparticles by the use of a microfluidic device is described. The microfluidic device comprises a first channel having a first inlet; a second channel having a second inlet; a third channel having a third inlet; and a main channel having a main inlet and an outlet. The first channel, second channel, and third channel all lead into the main channel. The method comprises the steps of injecting a solution of a metal/ligand into the first inlet, injecting a solution of a reducing agent into the second inlet, injecting a solvent comprised of an ionic liquid into the third inlet, and injecting an inert carrier into the main inlet. The solution of the metal/ligand, the solution of the reducing agent, the solvent and the inert carrier is combined together in the main channel, and the metal/ligand and the reducing agent are reacted for a time sufficient to form a metal nanoparticle.

Additional advantages and other features of the present disclosure will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the disclosure. The advantages of the disclosure may be realized and obtained as particularly pointed out in the appended claims.

As will be realized, the present disclosure is capable of other and different examples, and its several details are capable of modifications in various obvious respects, all without departing from the disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of a microfluidic device used to synthesize nanoparticles according to one example of the present disclosure.

FIG. 2 is a representation of a portion of a microfluidic device used to synthesize nanoparticles according to another example of the present disclosure.

FIGS. 3 a-b are a TEM micrograph and UV-Vis spectrum of nanoparticles synthesized according to one example of the present disclosure.

FIG. 4 is an XRD pattern and SAED pattern of nanoparticles synthesized according to one example of the present disclosure.

DETAILED DESCRIPTION

Illustrative examples are now discussed and illustrated. Other examples may be used in addition or instead. Details which may be apparent or unnecessary may be omitted to save space or for a more effective presentation. Conversely, some examples may be practiced without all of the details which are disclosed.

FIG. 1 shows a diagram of a microfluidic device used for the synthesis of monodisperse nanoparticles according to an example of the present disclosure.

The microfluidic device 100 shown in FIG. 1 includes a first channel 10 having a first inlet 11, a second channel 20 having a second inlet 21, a third channel 30 having a third inlet 31, and a main channel 40 having a main inlet 41 and an outlet 42. The first channel 10, second channel 20, and third channel 30 all lead into the main channel 40.

The microfluidic device 100 may be any suitable material for use in synthesis of nanoparticles. In some examples, the microfluidic device 100 is comprised of a polymeric material. The polymeric material may include silicon, for example, poly(dimethylsiloxane) (PDMS).

The microfluidic device 100 may be fabricated through a standard photolithography process. In some examples, the photolithography process consists of a cast PDMS layer bonded to a bare PDMS substrate. The PDMS stamp is cast onto the mold and cured at 65° C. for at least 2 hours to recover PDMS surface hydrophobicity.

In some examples, the first channel 10, the second channel 20, the third channel 30, and the main channel 40 have a hydrophobic coating. The hydrophobic coating may be any coating suitable for use in the microfluidic device may include a fluoropolymer or a silicon containing compound. For example, before use, the device may be silanized with trichloro(1H,1H,2H,2H-per-fluorooctyl)silane (97%, Sigma-Aldrich) for 20 min to make the first, second, third and main channel surfaces hydrophobic and prevent device fouling by AuNP adsorption on the channel walls. In other examples, the microfluidic device may be comprised of glass, silicon and polycarbonate.

The device may be any size suitable to allow for the formation of monodisperse nanoparticles. In some examples, the first channel 10, the second channel 20, the third channel 30, and the main channel 40 each have a width in a range of from about 100 μm to about 2000 μm, and have a depth in a range of from about 20 μm to about 200 μm. For example, the microfluidic device 100 of the present disclosure as shown in FIG. 1 measures 3×5 cm overall and the channel widths and depths are 600 and 95 μm, respectively, with a total channel volume of 7.6 μL.

The method forming the nanoparticles may comprise the steps of injecting a solution of a metal/ligand into the first inlet, injecting a solution of a reducing agent into the second inlet, injecting a solvent comprised of an ionic liquid into the third inlet, and injecting an inert carrier into the main inlet.

The metal/ligand solution may include a mixture of a metal and a ligand capable of stabilizing the metal. The metal may include any metal capable of forming monodisperse nanoparticles. In some examples, the metal may be at least one selected from gold, silver, cobalt, copper, platinum, palladium and ruthenium.

The ligand capable of stabilizing the metal may include an ionic liquid (IL). In certain examples, the IL may be an imidazolium based compound. Imidazolium based ILs act as a dual-function solvent system and stabilizing ligand for metal nanoparticles. ILs may be used as solvents for synthetic reactions in poly(dimethylsiloxane) (PDMS) microfluidic devices since they do not suffer from the PDMS incompatibility that limits the use of traditional organic solvents. Moreover, ILs are nonflammable and possess negligible vapor pressures, in addition to having the ability to stabilize metal nanoparticles because of their high ionic charge, high dielectric constant, and ability to form supramolecular hydrogen-bonded networks in the condensed phase, which may serve as structure directing networks for nanoparticle growth.

In some examples, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) may be used as the ligand. In other examples, 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide (BMIM-Tf2N) may be used as the ligand, since BMIM-Tf2N exhibits greater stability of the bistriflimide anion compared to tetrafluoroborate. Further, the hydrophobicity of BMIM-Tf2N, makes it easier to obtain in purer form.

In the second channel 20, the reducing agent added may be any suitable reducing agent capable of reacting with the metal/ligand to produce a nanoparticle. For example, a borohydride, such as sodium borohydride (NaBH4) is an acceptable reducing agent. In other examples, the reducing agent is an imidazolium based borohydride, such as 1-butyl-3-methylimidazolium borohydride (BMIM-BH4). Substituting BMIM-BH4 for NaBH4 may provide improved reducing agent solubility in the ionic liquid without the possibility of forming sodium-containing byproducts.

In the third channel 30, a solvent may be injected via the third input 31. The solvent should be one capable of solubilizing the metal/ligand and reducing agent, and allow for the reaction to produce nanoparticles to proceed. In some examples, the solvent may be used to insure that reagent mixing occurs only by interdiffusion between laminar streams, and not before the mixture of the metal/ligand and reducing agent reaches the main channel 40. In some examples, the solvent is 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4; 98+%, Alfa Aesar).

The inert carrier is used to carry the reactants, e.g., the metal/ligand and reducing agent, along the main channel 40 and out the outlet 42 where the products may be collected. The rates of injection of the metal/ligand, reducing agent, and solvent compared to the flow rate of the inert carrier may have an influence on the formation of the nanoparticles. Although more controlled than mixing in a macroscale batch reactor, flow focused laminar mixing within microchannels is diffusion limited, and concentration gradients can lead to polydispersity in nanoparticle syntheses. One solution to this problem is the use of droplet flows. Droplet flow microfluidic reactors allow for the generation of discrete droplets that are separated from one another by the inert carrier. The inert carrier may be immiscible with the metal/ligand, reductant and solvent.

In this configuration, mixing within the droplet is rapid and can be precisely controlled, unlike in conventional macroscale batch reactors where mixing is almost always turbulent and not well-defined. Droplet flows can eliminate concentration dispersion and maintain a constant ratio of reagents in all droplets. Convective mixing within these droplets has been shown to decrease the mixing time by 2 orders of magnitude as compared to diffusive mixing between coflowing laminar streams. The rate of mixing and type of mixing in discrete droplets separated by an immiscible carrier phase can be systematically tuned through varying the flow rates of the different phases.

In the present disclosure, the ratio of mixing of the ratio of the rate of injection of the inert carrier compared to the rate of injection of the solution of the metal/ligand, the solution of the reducing agent and the solvent is from about 2:1 to about 20:1. The ratio of the rate of injection of the solution of the metal/ligand to the solvent, the reductant to solvent, or the metal/ligand to reductant is from about 1:1 to about 3:1. In other examples, the ratio of the rate of injection of the metal/ligand to the solution of the reducing agent is from about 0.5:1 to about 2:1. These injection rates may allow for the formation of droplets as discussed above. For example, in the microfluidic device 200 as is shown in FIG. 2, the metal/ligand is injected into the first inlet 11 and passes through the first channel to combine with the reductant and solvent, which were passed through the second and third channels, respectively. The combination is then passed into the main channel 40 where the inert carrier, which was injected via the main inlet 41, is located. Due to the differences in viscosity and solubility, the combination forms a droplet 50, which comprises the combination of metal/ligand, reductant and solvent. In this droplet, the metal/ligand and the reducing agent are reacted for a time sufficient to form a metal nanoparticle. After formation of the nanoparticle, the nanoparticle is deposited through the outlet of the main channel 40 and out the outlet 42 (see FIG. 1).

The reaction time for formation of the nanoparticles is short. In some examples, the time of the reaction of metal/ligand and the reducing agent is from about 5 to about 60 seconds. The faster the reaction time, the more nanoparticles that may be formed.

The size of the metal nanoparticles formed has a narrow range. In some examples, the metal nanoparticle formed has a diameter of from about 3 nm to about 6 nm. The shape of the metal nanoparticle formed may be spherical.

EXAMPLES

As stated above, microfluidic-based syntheses of the present disclosure are a means to efficiently and reliably fabricate nanoparticles. The examples below are.

Example 1

A microfluidic device according to one example was fabricated through a standard photolithography process and consists of a cast PDMS layer bonded to a bare PDMS substrate. The device measures 3×5 cm overall and the channel widths and depths are 600 and 95 μm, respectively, with a total channel volume of 7.6 μL. Before use, the device was silanized with trichloro(1H,1H,2H,2H-per-fluorooctyl)silane (97%, Sigma-Aldrich) for 20 min to make the channel surface hydrophobic and prevent device fouling by gold nanoparticles (AuNP) adsorption on the channel walls. A stream of pure BMIM-BF4 was injected between the two reagent streams (HAuCl4/1-methylimidazole and NaBH4 in BMIM-BF4) to insure that reagent mixing occurred only by interdiffusion between laminar streams. This pure BMIM-BF4 stream and the two reagent streams were injected by syringe pump at a flow rate ratio of 5:9:9 via inlets I, II, and III, respectively. An inert polychlorotrifluoroethylene oil (Halocarbon 6.3 oil; River Edge, N.J.) was introduced via the main inlet 41 (see FIG. 1) at a flow rate of either 2070 μL/h or 7000 μL/h. These inert carrier oil flow rates defined two flow regimes—at the lower flow rate, the central IL stream remained continuous through the device while at the higher flow rate it broke up into droplets (the transition between flow regimes occurred at an inert oil flow rate of about 3000 μL/h). The reaction products were collected continuously from outlet 42 and quenched/precipitated into a reservoir containing ethanol.

The AuNPs were isolated by centrifugation (6000 rpm, 5 min) followed by washing twice with fresh ethanol to remove excess BMIM-BF₄. The isolated AuNPs were redispersed in hexanes and 1-dodecanethiol (5-10 μL/mL hexanes) by sonication, which gave red suspensions that were stable for months. The resulting AuNPs were analyzed by powder X-ray diffraction (XRD), selected area electron diffraction (SAED) and UV-Vis spectroscopy. The size, morphology, and size distribution of the resulting AuNPs were characterized by transmission electron microscopy (TEM).

Comparatively, the gold nanoparticles that we have obtained are monodisperse with a mean diameter of 4.38±0.53 nm (σ/μ_(d)=12.1%) and form 20 arrays as determined by transmission electron microscopy (TEM) (see FIG. 3 a). These particles were overwhelmingly spherical with only 15.0% having a roundness, defined as (4×particle area)/[π×(major axis length)²], less than 0.85. UV-Vis spectra of the deep red AuNP suspensions gave relatively narrow surface plasmon bands centered at d=518.5 nm, typical of AuNPs that are largely non-agglomerated (see FIG. 4). The XRD pattern of the AuNPs can be assigned to the face centered cubic (fcc) structure of gold (JCPDS no. 04-0784), with a lattice parameter of a=4.07 A˜that matches literature values. The crystallinity of the AuNPs was further confirmed by SAED analysis. The diffuse diffraction rings produced by an ensemble of nanoparticles were indexed to the characteristic {111}, {200}, {220}, and {311} diffraction planes of fcc gold (inset, FIG. 4), and the lattice parameter corroborates that calculated by powder XRD. Energy dispersive X-ray spectroscopic analysis suggests that the majority of the 1-methylimidazole and BMIM-BF₄ is displaced from the AuNP surface upon work-up with thiol, with the analyses for nitrogen and fluorine giving baseline integrated signal relative to that of sulfur.

Flow focusing by inert carrier oil has a dramatic effect on the size and morphological fidelity of the AuNPs. When the flow rate of the inert oil is decreased such that the IL stream no longer forms droplets, particles are larger (5.65 f 1.03 nm) and more polydisperse (v/μd=18.2%). The particles were also less spherical, with 23.4% having a roundness less than 0.85. In either flow-focused device geometry, the inert oil forces the two reagent streams, and the center BMIM-BF₄ stream separating them, down to narrower widths. As a result, the diffusion lengths needed for the two reagents to mix in the center BMIM-BF₄ stream are reduced. To illustrate the importance of flow focusing, an analogous microfluidic device was constructed without the additional carrier oil inlet (i.e., inlet 41 in FIG. 1). The three streams of BMIM-BF₄, HAuCl₄/1-methylimidazole, and NaBH₄ were introduced via syringe pump at flow rates of 500, 900, and 900 μL h-1 1 through inlets 11, 21, and 31, respectively. Laminar flow is observed under these conditions, with AuNP formation discernible further down channel as the center stream turns purple in color. Using this laminar flow device configuration, the AuNPs are more polydisperse and have a larger mean diameter (6.25 f 1.29 nm; σ/μd=20.6%), in addition to being more spheroidal in shape with 28.2% having a roundness less than 0.85. Thus, it is likely that the decreased interdiffusion distance between reagent streams in the fast inert carrier flow-focused geometry leads to faster mixing, which in turn constrains the nucleation burst, resulting in less polydisperse particles.

Example 2

Formation of Microfluidic Device:

A stable droplet formation of the viscous BMIM-Tf₂N ionic liquid within a continuous fluorocarbon oil phase, poly(chlorotrifluoroethylene)(PCTFE), was achieved by modifying the interior surfaces of preassembled PDMS devices with a fluoropolymer coating via a previously reported initiated chemical vapor deposition (iCVD) method. Briefly, the poly(1H,1H,2H,2H-perfluorodecyl acrylate-co-ethylene glycol diacrylate) coating is deposited in a vapor phase polymerization process where monomer molecules and initiator radicals polymerize via a free-radical chain mechanism on the interior surfaces of the preassembled channels. Coated devices performed with no signs of degradation or delamination for at least 24 h.

Microfluidic Synthesis of Au Nanoparticles.

Solutions of HAuCl₄ (10 mM), 1-methylimidazole (5 M), and BMIM-BH₄ (0.1 M) were prepared in BMIM-Tf₂N with stirring at 25° C. Equal volumes of HAuCl₄ and 1-methylimidazole solutions were thoroughly mixed before being introduced on device via syringe pump. Syringes and outlet tubing interfaced with the microfluidic device via PEEK tubing (I.D.=0.762 mm) and exited the device via silicon tubing (I.D.=1.02 mm). Reagent solutions of HAuCl₄/1-methylimidazole and BMIM-BH₄ were injected through inlets 11 and 21, respectively. A pure BMIM-Tf₂N buffer stream was injected between the two reagent streams via inlet 31. All dispersed phase reagents had a flow rate of 0.5 mL h⁻¹. The immiscible carrier oil, poly(chlorotrifluoroethylene) (PCTFE) was injected into the main channel 40 with a flow rate of 10 mL h⁻¹ via inlet 41. The samples exited the microfluidic device and were collected for 30 min in an empty collection tube (residence time=60 s) where they separated into two distinct phases and the oil phase was removed prior to workup. The AuNPs were precipitated by centrifugation after the addition of ethanol (4 mL). The colorless supernatant was replaced with fresh ethanol and the mixture was sonicated for 2 min using a probe sonicator fitted with a microtip at 50% duty cycle (Sonifier S-450A analog ultrasonic processor, Branson). The AuNPs were again isolated by centrifugation and finally redispersed in hexanes and 1-dodecanethiol (10-20 μL mL⁻¹ hexanes) with probe sonication for 1 min.

Microfluidic Synthesis of Ag Nanoparticles.

Solutions of AgBF₄ (40 mM), 1-methylimidazole (1.2 M), and BMIM-BH₄ (200 mM) were prepared in BMIM-Tf₂N with stirring at 25° C. Equal volumes of AgBF₄ and 1-methylimidazole solutions were thoroughly mixed before being introduced on device via syringe pump. Reagent solutions of AgBF₄/1-methylimidazole and BMIM-BH₄ were injected through inlets 11 and 31, respectively (see FIG. 2). Solutions containing the AgBF₄ were protected from light until before the reaction. A pure BMIM-Tf₂N buffer stream was injected between the two reagent streams via inlet 21. All dispersed phase inlets had a flow rate of 0.5 mL h⁻¹. The immiscible carrier oil was injected into the main channel with a flow rate of 10 mL h⁻¹ viainlet 1. The AgNPs were isolated by phase transfer whereby the AgNP dispersion in BMIM-Tf₂N was collected into an organic phase containing hexanes (2 mL), ethanol (2 mL), 1- dodecanethiol (50 μL), and trioctylamine (25 μL). The samples were collected for 30 min (residence time=2 min 45 s) and the colored organic layer containing AgNPs was transferred to a new centrifuge tube. The AgNPs were precipitated by centrifugation after the addition of methanol (3 mL). The colorless supernatant was replaced with ethanol and the mixture was sonicated for 2 min using a probe sonicator fitted with a microtip at 50% duty cycle. The AgNPs were again isolated by centrifugation and finally redispersed in hexanes (1-2 mL) with probe sonication for 1 min.

Batch Synthesis of Au and Ag Nanoparticles.

Solutions of HAuCl₄ (10 mM), 1-methylimidazole (5 M), and BMIM-BH₄ (0.1 M) were prepared in BMIM-Tf₂N with stirring at 25° C. Solutions of HAuCl₄ (0.25 mL) and 1-methylimidazole (0.25 mL) were thoroughly mixed. Thereafter, 0.5 mL of the BMIM-BH₄ solution was rapidly injected resulting in an immediate color change. After stirring for 1 min the AuNPs were precipitated by centrifugation with the addition of ethanol (4 mL). The colorless supernatant was replaced with fresh ethanol and the mixture was sonicated for 2 min using a probe sonicator fitted with a microtip at 50% duty cycle. The AuNPs were again isolated by centrifugation and finally redispersed in hexanes and 1-dodecanethiol (10-20 μL mL⁻¹ hexanes) with probe sonication for 1 min.

For the synthesis of AgNPs, solutions of AgBF4 (40 mM), 1-methylimidazole (1.2 M) and BMIM-BH4 (200 mM) were prepared in BMIM-Tf2N with stirring at 25° C. Solutions of AgBF4 (0.25 mL) and 1-methylimidazole (0.25 mL) in BMIM-Tf2N were thoroughly mixed in the absence of light. Thereafter, a solution of BMIM-BH4 in BMIMTf2N (0.5 mL) was rapidly injected resulting in a color change after ca. 10 s. The AgNPs were isolated by phase transfer whereby the AgNP dispersion in BMIM-Tf2N was collected into an organic phase containing hexanes (2 mL), ethanol (2 mL), 1-dodecanethiol (50 μL), and trioctylamine (25 μL). The colored organic phase containing AgNPs was separated and the AgNPs precipitated by centrifugation with the addition of methanol (3 mL). The colorless supernatant was replaced with ethanol and the mixture was sonicated for 2 min using a probe sonicator fitted with a microtip at 50% duty cycle. The AgNPs were again isolated by centrifugation and finally redispersed in hexanes (1-2 mL) with probe sonication for 1 min.

Characterization.

TEM was performed on a JEOL JEM-2100 electron microscope at an operating voltage of 200 kV, equipped with a Gatan Orius CCD camera. UV-Vis absorption spectra were collected on a Shimadzu UV-1800 spectrophotometer in dual beam mode using quartz cuvettes with 1-cm path lengths from nanoparticle dispersions in hexanes. NMR spectra were collected on a Varian VNMRS-500 2-Channel NMR spectrometer at 25° C. Viscosity was measured using a Cannon-Ubbelohde viscometer at room temperature. The /liquid mixture was composed of the reagent solutions for AuNP synthesis with the HAuCl₄ solution replaced by pure BMIM-Tf₂N to prevent gold nanoparticle formation.

The AuNPs synthesized on device under optimal droplet flow conditions were spherical and monodisperse with a mean diameter of 4.28±0.84 nm (n=54684), with average major and minor axis lengths of 4.78 and 4.13 nm, respectively.

The nanoparticles exhibited an ellipticity of 1.16, defined as the major axis length/minor axis length. The AuNPs produced in an analogous batch reaction were larger with a mean diameter of 5.52±0.98 nm (n=57 732) and average major and minor axis lengths of 6.09 and 5.18 nm, respectively. The AuNPs produced in the batch reaction possessed an ellipticity of 1.18.

The end-result AgNPs synthesized on device had a mean diameter of 3.73±0.77 nm (n=30 249) with major and minor axis lengths of 4.65 and 3.68 nm, respectively. Striking differences were observed for AgNPs synthesized in batch. Whereas well-defined spherical AgNPs were produced on device, the same conditions in batch produced large coral-like assemblies of very small AgNPs (<2 nm in diameter).

The present disclosure can be practiced by employing conventional materials, methodology and equipment. Accordingly, the details of such materials, equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific materials, structures, chemicals, processes, etc., in order to provide a thorough understanding of the disclosure. However, it should be recognized that the present disclosure can be practiced without resorting to the details specifically set forth. In other instances, well known processing structures have not been described in detail, in order not to unnecessarily obscure the present disclosure.

Only a few examples of the present disclosure are shown and described herein. It is to be understood that the disclosure is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concepts as expressed herein.

The components, steps, features, objects, benefits and advantages which have been discussed are merely illustrative. None of them, nor the discussions relating to them, are intended to limit the scope of protection in any way. Numerous other examples are also contemplated. These include examples which have fewer, additional, and/or different components, steps, features, objects, benefits and advantages. These also include examples in which the components and/or steps are arranged and/or ordered differently.

Unless otherwise stated, all measurements, values, ratings, positions, magnitudes, sizes, and other specifications which are set forth in this specification are approximate, not exact. They are intended to have a reasonable range which is consistent with the functions to which they relate and with what is customary in the art to which they pertain. 

What is claimed is:
 1. A method for manufacturing metal nanoparticles by the use of a microfluidic device, the microfluidic device comprising: a first channel having a first inlet; a second channel having a second inlet; a third channel having a third inlet; and a main channel having a main inlet and an outlet, wherein the first channel, second channel, and third channel all lead into the main channel, the method comprising the steps of: injecting a solution of a metal/ligand into the first inlet, injecting a solution of a reducing agent into the second inlet, injecting a solvent comprised of an ionic liquid into the third inlet, and injecting an inert carrier into the main inlet, combining the solution of the metal/ligand, the solution of the reducing agent, the solvent and the inert carrier together in the main channel, and reacting the metal/ligand and the reducing agent for a time sufficient to form a metal nanoparticle.
 2. The method of claim 1, wherein the microfluidic device is comprised of a polymeric material.
 3. The method of claim 2, wherein the polymeric material comprises silicon.
 4. The method of claim 3, wherein the polymeric material is poly(dimethylsiloxane).
 5. The method of claim 1, wherein the first channel, the second channel, the third channel, and the main channel each have a width in a range of from about 100 μm to about 2000 μm.
 6. The method of claim 1, wherein the first channel, the second channel, the third channel, and the main channel each have a depth in a range of from about 20 μm to about 200 μm.
 7. The method of claim 1, wherein the first channel, the second channel, the third channel, and the main channel have a hydrophobic coating.
 8. The method of claim 7, wherein the coating comprises a fluoropolymer.
 9. The method of claim 1, wherein the metal/ligand solution is comprised of a mixture of a metal and a ligand capable of stabilizing the metal.
 10. The method of claim 9, wherein the metal is at least one selected from the group consisting of gold, silver, cobalt, copper, platinum, and palladium.
 11. The method of claim 9, wherein the ligand is comprised of an ionic liquid.
 12. The method of claim 11, wherein the ionic liquid is an imidazolium based compound.
 13. The method of claim 1, wherein the reducing agent is an imidazolium based borohydride.
 14. The method of claim 1, wherein inert carrier comprises a hydrophobic liquid.
 15. The method of claim 14, wherein the hydrophobic liquid comprises a fluorocarbon.
 16. The method of claim 1, wherein a ratio of the rate of injection of the inert carrier compared to the rate of injection of the solution of the metal/ligand, the solution of the reducing agent and the solvent is from about 2:1 to about 20:1.
 17. The method of claim 16, wherein a flow rate of the inert carrier is about 1 mL/hour to about 10 mL/hour.
 18. The method of claim 1, wherein a flow rate of the inert carrier is such that when the solution of the metal/ligand, the solution of the reducing agent and the solvent are combined with the inert carrier in the main channel, a droplet comprised of metal/ligand, the reducing agent and the solvent is formed.
 19. The method of claim 1, wherein a ratio of the rate of injection of the solution of the metal/ligand to the solvent is from about 1:1 to about 3:1.
 20. The method of claim 1, wherein a ratio of the rate of injection of the solution of the reducing agent to the solvent is from about 1:1 to about 3:1.
 21. The method of claim 1, wherein a ratio of the rate of injection of the solution of the metal/ligand to the solution of the reducing agent is from about 0.5:1 to about 2:1.
 22. The method of claim 1, wherein the metal nanoparticle formed has a diameter of from about 3 nm to about 6 nm.
 23. The method of claim 1, wherein the metal nanoparticle formed has a spherical shape.
 24. The method of claim 1, wherein the time of the reaction of metal/ligand and the reducing agent is from about 5 to about 60 seconds.
 25. The method of claim 1, wherein after reacting the metal/ligand and the reducing agent, the nanoparticle formed is deposited through the outlet of the main channel. 